Astronomy Tea Talk

Massive galaxies have evolved dramatically in the last 10 billion years, undergoing dramatic changes to their sizes, shapes and stellar populations.  In the context of this evolution, I will argue that velocity dispersion is the most fundamental property of galaxies, especially when attempting to identify unique populations of galaxies across cosmic time.  Although directly measuring velocity dispersion is observationally expensive, we introduce inferred velocity dispersion as implied by the stellar mass, Sérsic index and effective radius of galaxies in the SDSS.  We calculate inferred velocity dispersions for galaxies in the UDS and NMBS Cosmos Surveys and derive the velocity dispersion function (VDF) out to z=1.5, both for the entire population and separately for star-forming and quiescent galaxies.  Remarkably, we find that overall the VDF is quite stable, providing evidence for inside-out galaxy growth.  Specifically, we find that galaxies with high velocity dispersions form early: the number density of both star-forming and quiescent galaxies remains roughly constant with time.  Additionally we find that the number of star-forming galaxies as a function of velocity dispersion is quite stable with time.   The only galaxy population showing strong evolution are quiescent galaxies with low inferred dispersions, for which number density increases by a factor of ~4 since z=1.5. We show that our results are qualitatively consistent with a simple model in which star-forming galaxies quench and are added to the quiescent population. In order to compensate for this migration, the velocity dispersions of star-forming galaxies must increase, with a rate that increases with dispersion. This model predicts that there must be a significant population of quenching and recently quenched galaxies at z>1.  Finally, I will present evidence for this model with the discovery of galaxies at z~1.5 with high velocity dispersions and strong Balmer lines.